1. Field of the Invention
The present invention generally relates to the field of instrumentation, and more specifically to an optical sensing device and a method thereof, based on Fabry-Perot type interferometry, for measuring a physical parameter such as a pressure, a temperature, the refractive index of a liquid, etc., and especially a strain in or a deformation of a body.
2. Description of the Prior Art
Strain sensors using optical fibers have evolved in almost all the fields involving strain sensing measurements of mechanical micro-deformations. In addition to traditional applications which are presently carried out by strain gages of resistive, piezoelectric or other types, the strain sensors using optical fibers find uses in new applications coming out from their development. For example, this is the case for smart skin sensors where the optical fibers, in reason to their small size, can be integrated within the structures to monitor.
One particular type of strain sensors using optical fibers is worthy of note. This is the Fabry-Perot type of strain sensors, which can be compared advantageously on one hand with the conventional electrical strain sensors, and on the other hand with the strain sensors using optical fibers found in the literature. The electrical strain sensors suffers however from their sensitivity to temperature and the supply current across them causing overheating. Their response are not linear, and they require frequent calibrations. Their output signal is moreover low (a few milivolts) and thus highly sensitive to bad connections and electrical or magnetic fields. Drift current may also occurs in damp conditions.
The properties inherent to optical fibers or devices overcome these drawbacks. For instance, sensors using optical fibers are immunized to electromagnetic fields, provide a better precision than traditional gages and resist to hard environment conditions.
A large number of techniques using optical fibers for strain measurements have been already proposed. Interferometric methods are almost the only ones providing precision, stability and dynamic ranges which satisfy most of the applications: on-board weighing systems for road vehicles, planes or others; systems dedicated to monitor the integrity of structures; etc.
Known in the art are the following documents: Davis U.S. Pat. No. 4,755,668, Jul. 5, 1988; MURPHY et al., "Quadrature phase-shifted, extrinsic Fabry-Perot optical fiber sensors", Optics Letters/Feb. 1991/Vol. 16, No. 4, pp. 273-275; LESKO et al., "Embedded Fabry-Perot fiber optic strain sensors in the macromodel composites", Optical Engineering/Jan. 1992/Vol. 31, No 1, pp.13-22; MURPHY, "Fabry-Perot fiber optic sensors in full-scale fatigue testing on an F-15 aircraft", Applied Optics/Jan. 1992/Vol. 31, No 4, pp.431-433. These documents relates to Fabry-Perot strain sensors using single mode optical fibers. Consequently, the light sources used are laser sources requiring to be stabilized with extreme precision. The measures, which are relative, are carried out by scanning whether the wavelength emitted by the laser or the physical length of an optical fiber acting as a reference, therefore increasing the complexity of the measurement device and reducing its stability. The measures are relative since only the interference fringes are counted with respect to a reference number, requiring therefore further computations to determine the sensed parameter.
Also known in the art are the following documents: HARTL et al., "Fiber optic temperature sensor using spectral modulation", SPIE/1987/Vol. 838; ID Systems, "Fiber-optic sensing of physical parameters", Sensors/1987/pp.257-261; SAASKI et al., "Measurement of physical parameters in composite materials using embedded fiberoptic sensors", SPIE/1989/Vol. 1170, pp.143-149. These documents relates to Fabry-Perot sensors and methods involving optical fibers and broadband light sources. By passing a light signal of known distribution through a Fabry-Perot interferometer subjected to the sensed physical parameter, and by analyzing with a specific electronic processing unit the spectrum of the light signal resulting from the interferometer, the sensed parameter can be determined. However, the analysis carried out by the processing unit is time consuming, without mentioning the substantial cost to manufacture such a processing unit.
Also known in the art is the document LEFEBVRE, "White light interferometry in optical fiber sensors", Proceedings 7th OFS conference, which reviews the applications of white light interferometry in the domain of optical fiber sensors. It proposes the use of a tilted Fizeau interferometer that yields to a spatial fringe pattern that can be easily analyzed to determine spectral characteristics of a light signal. It brings forward the idea of connecting a Michelson interferometer to the Fizeau interferometer for measuring a physical parameter, which in a theoretical point of view should be possible. But the way to obtain the desired results is far from explained in a technical point of view, without mentioning that the application or use of such a Michelson interferometer can be difficult depending on the situation requirements. Indeed, the mechanical stability required for operation of the Michelson interferometer is extremely hard to achieve, without mentioning difficulties with piece alignments. No Fabry-Perot interferometers have been proposed to achieve this because the author probably knew that with such a Fabry-Perot interferometer using multimode optical fibers there would have been, at that time, difficulties to obtain a light signal having enough intensity to produce an analyzable fringe pattern.
Yet known in the art are the following documents: U.S. Pat. No. 4,861,136, STONE et al., Aug. 29, 1989; European patent published under No 0,143,645, Mallinson et al., Jun. 5, 1985; SHABUSHNIG et al., "Formulation monitoring with a fiber optic refractive index sensor", Chemical Processing/Sept. 1988; LEE et al., "Fiber-optic Fabry-Perot temperature sensor using a low-coherence light source", Journal of Lightwave Technology/Jan. 1991/Vol. 9, No 1, pp. 129-134; DELISLE et al., "Application de la modulation spectrale a la transmission de l'information", Can. J. Phys./1975/Vol. 53, pp.1047-1053. These documents relate to various optical devices used for several purposes such as filters, communications, etc., but are whether too complicated, not suitable or not cost-attractive to be used for measuring physical parameters.
Therefore, an object of the present invention is to provide an optical sensing device based on a Fabry-Perot interferometric method, for measuring a physical parameter, which is simple, competitive to the electrical sensors, and cost-effective with comparison to the other sensors using optical fibers of the prior art.
It is a further object of the invention to provide such an optical sensing device with an excellent measurement precision and stability, a linear absolute response which do not requires additional computations before processing to the real determination of the physical parameter, and an adjustable high sensitivity and dynamic range to the physical parameter.
Still another object of the invention is to provide such an optical sensing device having a gaging portion with small dimensions, which can be used to measure several types of physical parameters, and that can be thermally auto-compensated.